The present invention relates to liquid droplet ejection systems and, more particularly, ink jet system and, even more particularly, to drop-on-demand ink jet systems.
Ink jet systems generally fall into two categoriesxe2x80x94continuous systems and drop-on-demand systems. Continuous ink jet systems operate by continuously ejecting droplets of ink, some of which are deflected by some suitable means prior to reaching the substrate being imprinted, allowing the undeflected drops to form the desired imprinting pattern. In drop-on-demand systems, drops are produced only when and where needed to help form the desired image on the substrate.
Drop-on-demand ink jet systems can, in turn, be divided into two major categories on the basis of the type of ink driver used. Most systems in use today are of the thermal bubble type wherein the ejection of ink droplets is effected through the boiling of the ink. Other drop-on-demand ink jet systems use piezoelectric crystals which change their planar dimensions in response to an applied voltage and thereby cause the ejection of a drop of ink from an adjoining ink chamber.
Typically, a piezoelectric crystal is bonded to a thin diaphragm which bounds a small chamber or cavity full of ink or the piezoelectric crystal directly forms the cavity walls. Ink is fed to the chamber through an inlet opening and leaves the chamber through an outlet, typically a nozzle. When a voltage is applied to the piezoelectric crystal, the crystal attempts to change its planar dimensions and, because the crystal is securely connected to the diaphragm, the result is the bending of the diaphragm into the chamber. The bending of the diaphragm effectively reduces the volume of the chamber and causes ink to flow out of the chamber through both the inlet opening and the outlet nozzle. The fluid impedances of the inlet and outlet openings are such that a suitable amount of ink exits the outlet nozzle during the bending of the diaphragm. When the diaphragm returns to its rest position ink is drawn into the chamber so as to refill it so that it is ready to eject the next drop.
Thermal bubble systems, although highly desirable for a variety of applications, suffer from a number of disadvantages relative to piezoelectric crystal systems. For example, the useful life of a thermal bubble system print head is considerably shortened, primarily because of the stresses which are imposed on the resistor protecting layer by the collapsing of bubbles. In addition, because of the inherent nature of the boiling process, it is relatively difficult to precisely control the volume of the drop and its directionality. As a result, the produced dot quality on a substrate may be less than optimal.
Still another drawback of thermal bubble systems is related to the fact that the boiling of the ink is achieved at high temperatures, which calls for the use of inks which can tolerate such elevated temperatures without undergoing either mechanical or chemical degradation. As a result of this limitation, only a relatively small number of ink formulations, generally aqueous inks, can be used in thermal bubble systems.
These disadvantages are not present in piezoelectric crystal drivers, primarily because piezoelectric crystal drivers are not required to operate at elevated temperatures. Thus, piezoelectric crystal drivers are not subjected to large heat-induced stresses. For the same reason, piezoelectric crystal drivers can accommodate a much wider selection of inks. Furthermore, the shape, timing and duration of the ink driving pulse is more easily controlled. Finally, the operational life of a piezoelectric crystal driver, and hence of the print head, is much longer. The increased useful life of the piezoelectric crystal print head, as compared to the corresponding thermal bubble device, makes it more suitable for large, stationary and heavily used print heads.
Piezoelectric crystal drop-on-demand print heads have been the subject of much technological development. Some illustrative examples of such developments include U.S. Pat. Nos. 5,087,930 and 4,730,197, which are incorporated by reference in their entirety as if fully set forth herein and which disclose a construction having a series of stainless steel layers. The layers are of various thicknesses and include various openings and channels. The various layers are stacked and bonded together to form a suitable fluid inlet channel, pressure cavity, fluid outlet channel and orifice plate.
The systems disclosed in the above-referenced patents illustrate the use of a fluid inlet channel having a very small aperture, typically, 100 microns or less. The use of a very small aperture is dictated by the desirability of limiting the backflow from the ink cavity during ejection of a drop but is problematic in that the small aperture is susceptible to clogging during the bonding of layers as well as during normal operation of the print head.
The construction disclosed in the above-referenced patents requires the very accurate alignment of the various layers during manufacture, especially in the vicinity of the small apertures which form portions of the fluid path. Furthermore, the openings in the orifice plate which form the outlets of the various flow channels have sharp edges which could have adverse effects on the fluid mechanics of the system.
Additionally, the techniques used in forming the openings in the orifice plate, which typically include punching, chemical etching or laser drilling, require that the thickness of the orifice plate be equal to, or less than, the orifice diameter which is itself limited by resolution considerations to about 50 microns.
Finally, any air bubbles trapped inside the flow channel cannot easily be purged and, because the bubbles are compressible, their presence in the system can have detrimental effects on system performance.
According to the present invention there is provided a liquid droplet ejection device, comprising: (a) a plurality of liquid ejection nozzles; (b) a liquid supply layer including porous material, the liquid supply layer featuring holes related to the nozzles; and (c) a plurality of transducers related to the holes for ejecting liquid droplets out through the nozzles.
In preferred embodiments of devices according to the present invention, the porous material includes sintered material, most preferably, sintered stainless steel.
According to one embodiment of the present invention, the transducers are piezoelectric elements, the nozzles are the outlets of capillaries and the device further comprises: (d) a deflection plate, the piezoelectric elements being connected to the deflection plate; and (e) a liquid cavity layer formed with cutouts therethrough, the cutouts being related to the piezoelectric elements, the liquid cavity layer adjoining the deflection plate, the liquid cavity layer adjoining the liquid supply layer, the holes of the liquid supply layer being related to the cutouts, the capillaries located in the holes, the liquid supply layer being configured so that liquid is able to flow from the porous material into the cutouts.
According to another embodiment of the present invention, the liquid cavity layer is omitted and the deflection layer directly adjoins the liquid supply layer.
According to yet other embodiments of the present invention, the nozzles are formed by an orifice plate which adjoins the liquid supply layer, which may, in turn, adjoins the deflection plate or the liquid cavity layer, when present.
According to other embodiments of the present invention, the transducers are heat elements and droplet ejection is effected by the thermal bubble method, rather than through the use of piezoelectric elements.
The ejection of ink drops using a device according to one embodiment of the present invention is accomplished as follows: A pressure pulse is imparted to a volume of ink in an ink cavity through the deflection of a thin deflection plate, or diaphragm, located on top of the ink cavity. The plate is deflected downward by the action of a piezoceramic crystal whenever a voltage is applied across its electrodes, one of which is in electrical contact with the usually metallic deflection plate.
The pressure pulse created by the downward bending of the deflection plate drives the ink towards and through an outlet, preferably a glass capillary having a convergent nozzle at its outlet end, causing the ejection of a drop of a specific size.
When the piezoelectric crystal is de-energized, it returns to its equilibrium position, reducing the pressure in the ink cavity and causing the meniscus at the outlet end of the glass capillary to retract.
The retracted meniscus generates a capillary force in the glass capillary which acts to pull ink from an ink reservoir into the ink cavity and into the glass capillary. The refilling process ends when the meniscus regains its equilibrium position.
In alternative embodiments of devices of the present invention there are provided systems similar to those presented above but which, instead of relying on piezoelectric elements and a deflecting plate, features heating elements which serve to boil the ink, thereby causing its ejection.
A key element in print heads according to the present invention is the presence of porous material which is in hydraulic communication with both the ink reservoir and the individual ink cavities. Preferably, the glass capillaries are embedded in openings in the porous material. The porous material preferably also defines part of the walls of the ink cavities.
Proper selection of the porous material makes it useful as a filter, serving to prevent any foreign particles which may be present in the ink from reaching the nozzles and possibly blocking them.
It will be readily appreciated that in order to achieve high drop ejection rates, the time required to refill the ink cavity following ejection of a drop must be as short as possible. The refilling time can be reduced by reducing the restriction to flow into the ink cavity. However, reduction of the restriction to inflow tends to increase the adverse effects of cross talk, i.e., the undesired interactions between separate ink cavities.
The optimization of the system in terms of the conflicting requirements of low cross talk and high refill rate can be effected through the judicious selection of a porous material having optimal characteristics for the intended application, taking into account, in addition, the viscosity of the ink and the nozzle geometry. The important characteristics of the porous material include the pore size and the permeability to flow (together referred to as xe2x80x9cmicron gradexe2x80x9d), as well as the macro and micro geometries of the porous material.
As stated above, the optimal balance between the in-flow of ink into the ink cavity and its out-flow from the cavity is also affected by the ink viscosity and nozzle dimensions. The lower the viscosity of the ink, the faster is the refilling rate of the ink cavity but the more pronounced is the cross talk between separate cavities. Also, the smaller the outlet nozzle diameter, the more pronounced is the capillary action of the nozzle and hence, the higher is the refilling rate.
Ink jet print heads are generally designed so that the dimensions of the ink channels into and out of the ink cavity are such that the channels have acoustic impedances which are optimal for a specific ink of a given viscosity and for a specific nozzle diameter. If it is desired to use a print head with a different nozzle diameter and/or with a different viscosity ink, the print head channels must be redesigned to accommodate the new nozzle diameter and/or different viscosity ink.
By contrast, use of a porous material according to the present invention, makes it possible to preserve the same print head geometry and structure even when ink of a different viscosity and/or when a different nozzle geometry are to be used. The optimization of the acoustic impedances of the channels can be effected merely through the proper selection of a suitable porous material having suitable characteristics, such as a suitable micron grade.
Apart from the ability to optimize the print head without the need to redesign the flow channels, use of porous materials according to the present invention eliminates the small, and easily clogged, ink inlet apertures leading to the ink cavities.
Still another advantage offered by the use of the porous material according to the present invention is the materials ability to act as a filter, thereby reducing, or even completely obviating, the need for special filtration of the in-flowing ink.
Finally, the fabrication of print heads including porous material according to the present invention can be effected using simple production techniques without the need for complex and expensive micro-machining.